Digital cameras include a two-dimensional pixel arrays. A single pixel can include one or more photo-detectors as well as multiple transistors. Typical photo-detectors include photodiodes, phototransistors, photo-gates, hole accumulation diodes, pinned diodes, avalanche diodes, buried accumulation and transfer layer devices.
Various prior art pixels are known. The most commonly used pixels are either CCD pixels or CMOS pixels. Prior art CMOS pixels and two dimensional CMOS arrays are illustrated in the following U.S. patents which are incorporated herein by reference: U.S. Pat. No. 6,777,660 of Lee, U.S. Pat. No. 6,762,401 of Lee, U.S. Pat. No. 6,567,495 of Harada, U.S. Pat. No. 6,750,912 of Tennant et al., U.S. Pat. No. 6,697,111 of Kozlowski et al., U.S. Pat. No. 6,665,013 of Fossum et al., U.S. Pat. No. 6,587,142 of Kozlowski et al., U.S. Pat. No. 6,538,245 of Kozlowski, U.S. Pat. No. 6,532,040 of Kozlowski et al., U.S. Pat. No. 5,892,540 of Kozlowski et al., U.S. Pat. No. 5,238,276 of Dhuse et al., U.S. Pat. No. 6,326,230 of Pain et al., and European patent application EP1881692A2 titled “Methods for reading a pixel and for writing to a pixel and a device having pixel reading capabilities and pixel writing capabilities” which is incorporated herein by reference.
The pixels that form the pixel array are usually arranged in columns. Pixels that belong to the same pixel column may share various control and feedback signals.
The pixel columns are usually activated in a pipeline manner—while some pixel columns are reset other pixel columns are read ad yet other pixel columns undergo an integration phase. Typically, there is a relatively long time gap between the reset phase of a certain pixel column to a read phase of that pixel column.
Reference voltages are provided to pixels and to pixel readout circuits during the reset, read and integration phases. These reference voltages can affect the outcome of the reset and read phases, especially when applying correlated double sampling techniques.
Highly stable reference voltage sources are relatively expensive and their complexity can increase as a function of the number of pixel columns. Unstable reference voltage sources can output an unstable reference voltage that can vary between a reset phase of a pixel column to a read phase of that pixel column.
There is a need to provide efficient methods to provide reference signals to groups of pixels and to pixel readout circuits.